It’s a tantalizing prospect: the notion that two lanky British stars of Tumblr fantasies could appear together in the same TV series. But how likely is it, really, that Tom Hiddleston will be cropping up on Benedict Cumberbatch’s series Sherlock this season? Many in the fandom seem convinced it’s a done deal, while Hiddleston himself has flatly denied the rumor. In other words, we have a juicy mystery on our hands. So why not throw on a deerstalker hat, light up a handy meerschaum pipe, and do our best impression of the great detective himself in order to get to the bottom of this case? Will Tom Hiddleston show up just in time for the Season 4 (and possible series) finale? Let’s examine the clues.

Rumors have been circulating—for a long time now—that Hiddleston could conceivably play the third Holmes brother: a non-canonical invention called Sherrinford Holmes. Non-canonical stories are, essentially, fan fiction, and there are hundreds of books, plays, films, and other tales not written by Sir Arthur Conan Doyle that are set in the world of Holmes. In this case, when author William S. Baring-Gould wrote a fictional biography of Holmes, he included a second brother named Sherrinford. Why? Because of the “other brother” theory, which goes like this: if Mycroft had been the eldest heir of the landed Holmes family in 19th-century England, he would have been obligated to manage the family estate—and wouldn’t have been free for a government job. This fictitious Sherrinford frees up both Mycroft and Sherlock to go gallivanting on adventures. (Sherrinford Holmes, by the way, was a name Doyle considered before settling on Sherlock.)

Mark Gatiss’s character, Mycroft, first hinted at the existence of a third Holmes sibling back in Season 3. There was the obvious nod in the finale to “the other one,” whom Mycroft seemingly failed to use his governmental influence in order to protect.

But there was also this intriguing reference earlier in the season. Sherlock complains that Mycroft made him think he was the stupid brother when they were growing up.

“Both of us” could refer to Mycroft and Sherlock. Or it could refer to Mycroft and another sibling. If that’s the case, “Sherrinford” would have to be at least as smart as Mycroft, if not smarter.

When appearing at Comic-Con over the summer, Sherlock co-creator Gatiss added fuel to the Sherrinford speculation by revealing that name as a tantalizing clue for Episode 3 of Season 4. (Episode 1’s clue, “Thatcher,” refers to the plaster busts Sherlock investigates, while Episode 2’s clue, “Smith,” is a reference to the character played by Toby Jones.)

Speculation that Sherrinford could be played by Hiddleston pre-dated that Comic-Con clue, but Gatiss (probably intentionally) threw fuel on the fire by tweeting out the following photo.

“Blud” is U.K. slang for mate, but it obviously invokes “blood” and brothers. In fact, Sherlock uses the word “blood” to refer to Mycroft in Season 3, Episode 1.

But it’s worth remembering that Gatiss and Hiddleston are mates. The pair starred opposite each other in a 2014 stage production of Coriolanus.

It’s also worth remembering that Gatiss is well aware of the Hiddleston fan theory—and is also fond of messing with overly invested Sherlock fans. (Season 3, Episode 1 is essentially one long, winking nod to all the fan theories that sprung up around Sherlock’s non-fatal tumble from a hospital roof.) Tongue firmly in cheek, the BBC Store tweeted out a fun video of footage cut together from The Night Manager and Sherlock to demonstrate what Hiddleston on Sherlock would look like. The account has since mysteriously deleted the video. Perhaps because it hit too close to home?

At Comic-Con, the Nerdist’s Alicia Lutes asked Gatiss and co-creator Steven Moffat directly about the Sherrinford theory, wondering if they would have Hiddleston on Sherlock if they could. “Who says we haven’t?” Gatiss responded before cackling and adding, “I’m just f—cking with you.”

Cumberbatch was also asked about his mate Hiddleston at Comic-Con—though that was in the context of Marvel. The pair of friends—who have appeared together on-screen, in 2011’s War Horse—seem destined to meet up again as Loki and Stephen Strange in the Marvel Cinematic Universe. And that meeting may potentially come as early as 2017’s Thor: Ragnarok. Cumberbatch clearly wasn’t about to confirm or deny any future collaborations with Hiddleston, Sherlock, or otherwise, but he answered with a coy smile, “We’ll see. Stuff could happen.”

While playing coy or directly messing with fans might work for Cumberbatch and Gatiss, Hiddleston has taken a different approach. Back in April of last year—before the Comic-Con flurry—he directly addressed and shut down the Sherlock rumors. “I’ve never been approached to be in Sherlock,” he said, bemused. “I don’t understand where that rumor has come from.”

“Ah!” some fans may cry. He hadn’t been approached then, but what about now? Well, that’s a very good question. What is going on now with the Sherrinford theory? The name got a mention in Season 4, Episode 1—but note the phrasing. In an episode where Gatiss and Moffat turned Arthur Conan Doyle’s town of Norbury into a character named Vivian Norbury, could we be seeing the reverse? In this case, could “Sherrinford” be a place that Mycroft is calling?

Replace “Sherrinford” with “Buckingham Palace,” and the meaning stays the same. In other words, there are no limits to the ways Gatiss could be trolling fans. Many sharp viewers have also latched onto the phrasing of the show’s original third sibling reference. Note that Mycroft says “the other one,” which leaves the door open for a Holmes sister. The show already trod this path in the series premiere, when Sherlock mistakenly identified John’s sibling “Harry” as a brother when, in fact, she was Watson’s lesbian, alcoholic sister. Though “Harry” is a far more believably gender-neutral name than “Sherrinford.”

So if Sherrinford is indeed a Holmes brother—not a sister, not a place—then how reasonable is it to expect that character to be played by Tom Hiddleston? The whole reason Baring-Gould invented Sherrinford was to create an older brother for Mycroft and Sherlock. Hiddleston (35) is younger than both Cumberbatch (40) and Gatiss (50). For reference: Steven Moffat (55) once jokingly saidhe would be up for playing “the other one.”

Then again, the 19th-century rules of the landed gentry—that the eldest brother would inherit/have to look after the Holmes estate—don’t really apply to this modernized version of Sherlock. For one thing, Mycroft and Sherlock’s father is still alive. Also, the Holmes parents—whom we met in Season 3—don’t strike me as the landed gentry type. Their mother is an eccentric genius who wrote math books and both parents came across more middle class than the country squires of Doyle’s stories. (And, no, Sherlock’s lavish “mind palace” is not the same as the Holmes ancestral home.)

So, sure, Sherrinford could be a younger brother here. And Gatiss implies that whoever plays him should bear some resemblance to himself and Cumberbatch. When fans guessed that the actor Toby Jones could be playing the potential third brother, Gatiss replied that Jones “doesn’t look like a member of the family.” Whatever vague resemblance there may be between Gatiss and Cumberbatch (tall and slender?), Hiddleston certainly fits the bill.

Whatever or whoever Sherrinford is, we’ll find out in the fourth season finale—a.k.a. the 13th episode of the series.

It’s worth noting that the BBC and PBS are being especially tight-lipped about this season of Sherlock, and have refused to give advance screeners out to press. There was a dramatic plot twist to hide in Episode 1, sure—but could something even bigger and blonder be coming? If not, all of Gatiss’s teasing may have put him in a bit of a tight spot. If Sherrinford shows up and he isn’t played by Tom Hiddleston, expectant fans might have trouble embracing anyone else in his place. (Imagine if he’s Moffat instead.) Sherlock fans are forgiving, but a Hiddleston red herring? That might be too much, even for a scamp like Mark Gatiss.